Origins of major human infectious diseases N.D. Wolfe, C.P. Dunavan, J. Diamond Nature 2007
Origins of major human
infectious diseases
N.D. Wolfe, C.P. Dunavan, J. Diamond
Nature 2007
Questions of interest
• Where did these major infectious diseases come from?
• Why do most of them originate from the Old World?
• How does a disease progress from existing exclusively in animal pops into human pops?
• Focus: unicellular microbial pathogens
Diseases considered in the study:
▫ 8 temperate: hepatitis B, influenza A, measles, pertussis, rotavirus A, syphiis, tetanus and TB
▫ 9 tropical: AIDS, Chagus, cholera, DHF, E. and W. African sleeping sickness, malaria, leishmaniasis
▫ 8 others: diphtheria, mumps, plague, rubella, smallpox, typhoid and typhus, tropical yellow fever
Animal pathogen human pathogen
in 5 evolutionary stages
• 1. Microbe is present in animals but not humans
• 2. Animal pathogen is transmitted to humans but not between humans (anthrax, rabies, etc)
• 3. animal pathogens have a few cycles of secondary transmission in humans but soon die out (ebola, Marburg, etc)
Prof. Durham’s ebola
• Stage 4:
▫ Disease exists in animals & humans can get infected through contact w/ the sylvatic cycle
▫ but also, the disease can be transmitted between humans w/o animal hosts
▫ Three substages:
� a) sylvatic cycle more important than direct human to human spread (Chagas disease, yellow fever)
� b) both sylvatic and direct transmission important (dengue)
� c) spread mostly through humans (typhus, influenza, cholera)
• Stage 5: pathogen is exclusive to humans
▫ Malaria, measles, mumps, rubella, smallpox and syphilis
▫ 2 ways pathogen became confined to humans:
� A)ancestral pathogen already present in the common ancestor of chimps and humans split
� B) animal pathogen recently colonized humans and evolved a separate human form
measles
Temperate/Tropical zone differences
• Greater proportion of diseases are transmitted by insect vectors in tropics than temperate zones
• Higher proportion of temperate diseases confer lasting immunity (upon survival) than in tropical regions
• Animal reservoirs are more common in tropics & an environmental reservoir is more common in temperate
• Temperate infectious diseases tend to be more acute (a few days duration) rather than chronic
• Tropical diseases are more chronic, some last weeks, months and years
• There are more stage 5 diseases in temperate zones
• There are more ‘crowd epidemic diseases’ in the temperate zones
Pathogen origins• >half of temperate diseases are from domestic animals
• Rise of agriculture helped build up crowds of people and livestock living close to each other
• Fewer tropical diseases originate from domestic animals
• Half of the tropical diseases have wild non human primate origins, compare to only one temperate
• Most diseases with animal origins are from warm-blooded animals
One exception to the last point
Geographic origins• Majority of 25 major pathogens considered originate from the Old World
▫ Historical significance in the colonization of the New World
▫ Why? : “More temperate diseases arose in the Old world than new World because far more animals that could furnish ancestral pathogens were domesticated in the Old World.”
▫ Human density: Agriculture led to higher human population densities infectious diseases thrive
Diamond is one of the authors of the paper
Future research and
steps to be taken
• 2 avenues
▫ Continue clarifying origins of existing diseases
▫ Increase surveillance for early detection of new diseases: global early warning system
Photo credits
• http://www.afip.org/Departments/infectious/mp/images/72main.jpg
• http://webs.wichita.edu/mschneegurt/biol103/lecture17/Ebola_EM.jpg
• http://katelewis.files.wordpress.com/2007/12/ebolaplush.jpg
• http://www.nlm.nih.gov/medlineplus/ency/images/ency/fullsize/2943.jpg
• http://weblogs.variety.com/photos/uncategorized/2007/11/18/cholera2.jpg
• http://www.skeptic.com/Merchant2/graphics/audio_video/av559_lg.jpg
• http://www.crimethinc.com/tools/downloads/preview_big/surveillance_big.gif
• The End• Thank you for staying awake!